A holographic stereogram is produced by, for example, using multiple images of a subject sequentially picked up from different observation points as original images and sequentially exposure-recording these original images to a single hologram recording medium as strip-like or dot-like element holograms.
For example, a holographic stereogram having parallax information only in the lateral direction is produced by sequentially displaying a plurality of original images 101a to 101e of a subject 100 sequentially picked up from different observation points in the lateral direction, on a display unit in a holographic stereogram producing device having a predetermined optical system, and then sequentially exposure-recording interference fringes generated by interference between object light modulated by casting a laser beam on the displayed images and reference light, as strip-like element holograms to a hologram recording medium 102, as shown in FIG. 17.
In the holographic stereogram thus produced, image information obtained by sequential image pickup from different observation points in the lateral direction is sequentially recorded in the lateral direction as strip-like element holograms. Therefore, if an observer at a certain position sees this holographic stereogram with one eye, an aggregate of image information recorded as a part of the element holograms is identified as a two-dimensional image. If the observer sees this holographic stereogram with one eye at a different position, an aggregate of image information recorded as another part of the element holograms is identified as another two-dimensional image. Therefore, if the observer sees the holographic stereogram with both eyes, the exposure-recorded image is identified as a three-dimensional image because of the parallax between the right and left eyes.
An application of such a holographic stereogram is, for example, a printer system made up of a combination of a shooting device which picks up images of a subject and generates a parallax image string and a printer device which outputs a holographic stereogram or a hologram as a printed matter as in the above-described holographic stereogram producing device, as described in Akira Shirakura, Nobuhiro Kihara and Shigeyuki Baba, “Instant holographic portrait printing system,” Proceeding of SPIE, Vol.3292, pp.246-253, January 1998, and Kihara, Shirakura and Baba, “High-speed hologram portrait printing system,” Three-Dimensional Image Conference 1998, July 1998, and so on. Such a printer system can provide services from shooting of a subject to printing of the result of shooting, in the same place.
In producing the above-described holographic stereogram, a so-called film-coated recording medium formed by holding a photopolymer layer 202 made of photopolymerization-type photopolymer between a base film 201 and a cover film 203 is used as a hologram recording medium 200, as shown in FIG. 18. The hologram recording medium 200 having such a multilayered film shape enables direct exposure-recording on the photopolymer layer 202 without having to replace it by glass or the like in producing the holographic stereogram. Therefore, it is very easy to handle.
As the base film 201 and the cover film 203 of such a hologram recording medium 200, polyethylene terephthalate (hereinafter referred to as PET) films are mainly used. This is because of various requirements in the process of manufacturing the hologram recording medium 200. Specifically, in the hologram recording medium 200, the chemical resistance of the base film 201 and the cover film 203 is required in the process of applying the photopolymer layer 202. Moreover, it is required that the photopolymer layer 202, and pigments, sensitizer and the like contained in the photopolymer layer 202 should not react or diffuse. Therefore, in the hologram recording medium 200, PET films are used as films that meet these requirements. The present applicant have verified this by comparative experiments with hologram recording media having base films and cover films of other materials.
It is known that the exposure-recorded holographic stereogram images of the holographic stereogram have uneven brightness depending on a hologram recording medium to be used. As a cause of this, the coherence of a laser beam cast on the hologram recording medium 200 in producing the holographic stereogram is considered. As an element affecting this coherence, double refraction caused by the base film 201 and the cover film 203 of the hologram recording medium 200 is considered. That is, the above-described PET film has double refraction and this double refraction of the PET film is considered to adversely affect the coherence of the laser beam.
FIG. 19 shows the concept of double refraction due to the PET film. For example, incident light I in a linearly polarized state with its direction of light wave oscillation indicated by an arrow aa in FIG. 19 is affected by double refraction when passing through a PET film PF. Therefore, exit light E passed through the PET film PF is typically in an elliptically polarized state with its direction of light wave oscillation indicated by an arrow bb in FIG. 19. Since the elliptically polarized state of the exit light E varies depending on the thickness, the manufacturing method and the direction of cutting of the PET film PF, it is difficult to uniformly manufacture the PET film PF with high precision. Moreover, the elliptically polarized state of the exit light E also varies depending on the polarization angle of the linearly polarized incident light I.
The brightness of the holographic stereogram images is decided by the material characteristics such as the percentage modulation of refractive index and the thickness of the hologram recording medium 200 and also depends on oscillations from outside at the time of exposure-recording and the coherence of a laser beam cast on the hologram recording medium 200 as described above. The coherence of the laser beam is affected by the polarization states of object light and reference light as well as the coherence distance of a laser light source to be used. As for the polarization states of object light and reference light, it is ideal that both object light O and reference light R cast on the hologram recording medium 200 are linearly polarized light having the same direction of light wave oscillation indicated by arrows cc and dd as shown in FIG. 20. Particularly, it is confirmed that the highest coherence is obtained when the reference light R is S-polarized light. It should be noted that the object light O and the reference light R have to be linearly polarized light in the layer where the holographic stereogram is actually recorded, that is, the photopolymer layer 202 of the hologram recording medium 200.
However, in the hologram recording medium 200 having the multilayer structure shown in FIG. 18, even if both object light and reference light incident on the hologram recording medium 200 are linearly S-polarized light, the object light and the reference light will not be linearly S-polarized light when reaching the photopolymer layer 202 because of double refraction due to the base film 201 and the cover film 203, which are made of PET films.
Thus, in the holographic stereogram, changes from the initial polarization states of the object light and the reference light due to double refraction due to the base film 201 and the cover film 203 adversely affect the coherence. As a result, the exposure-recorded holographic stereogram images are darkened.